1. Field of the Invention
The present invention relates to a lens control device suitable for use in an image pickup apparatus such as a video camera.
2. Description of Related Art
Conventionally, an image pickup apparatus such as a video camera has an image sensor using a solid-state image pickup element such as a CCD, in its image forming plane. In general, the larger the number of pixels per image pickup element such as a CCD, the larger the size of the image pickup element and hence the higher the cost of the same.
In terms of these considerations, numerous image pickup apparatuses for general domestic use employ a so-called 1/4-inch size CCD which is approximately 4 mm in diagonal length or a so-called 1/3-inch size CCD which is approximately 6 mm in diagonal length. These CCDs generally have 300,000 to 400,000 pixels.
Since the image size of such image pickup apparatus is small compared to 43 mm which is the diagonal length of the image size of a silver-halide camera using a so-called 135 film, it is possible to greatly reduce the size of a lens for the image pickup apparatus compared to the lens of the silver-halide camera if both lenses have the same angle of view.
In practice, video cameras using 1/4-inch size CCDs are generally provided with zoom lenses having a zoom ratio of 10 and an overall length of approximately 50 mm.
A best known type of zoom lens for such a video camera includes four lens groups which are arranged in the order of positive refractive power, negative refractive power, positive refractive power and positive refractive power as viewed from a subject side. Incidentally, zoom lenses of various other types using different lens-group arrangements have been put into practice to meet various specifications.
First of all, for example, the structure of the lens barrel of a representative zoom lens which includes four lens groups will be described below. Four lens groups constituting a zoom lens include a fixed front lens, a variator lens group which moves along an optical axis to effect a magnification varying operation, a fixed afocal lens, and a focusing lens group which moves along the optical axis to effect a focusing operation and maintain the position of a focal plane during a magnification varying operation.
FIG. 15 is a block diagram showing the electrical construction of the camera body of a conventional image pickup apparatus. In FIG. 15, a zoom lens includes four lens groups 201a to 201d. The lens group 201a constitutes a fixed front lens, the lens group 201b constitutes a variator lens group (or variator) which varies magnification, the lens group 201c constitutes a fixed afocal lens, and the lens group 201d constitutes a focusing lens group which effects focusing and also plays the role of a compensator for maintaining an image of a subject located at the same distance, in an in-focus state in an image forming plane during a magnification varying operation accompanied by the movement of the variator lens group 201b.
A solid-state image pickup element 221 such as a CCD is disposed in the image forming plane. A drive source 222 for the variator lens group 201b includes the motor 206, the gear train 207 interlocked with the motor 206, the screw rod 208 and the like. A drive source 223 for the focusing lens group 201d is composed of a stepping motor 212 or the like. Incidentally, the zoom driving source 222 may also be composed of a stepping motor similarly to the drive source 223 for the focusing lens group 201d.
Reference numeral 224 denotes an iris driving source, reference numeral 225 denotes a zoom encoder, and reference numeral 227 denotes a focus encoder. If stepping motors are used as the respective drive sources 222 and 223, each of the zoom encoder 225 and the focus encoder 227 is generally arranged to continuously count the number of operation pulses which are inputted to the corresponding one of the stepping motors after the corresponding lens group 201b or 201d starts to move from its origin position to which the lens group 201b or 201d is previously moved at an initial preparatory stage of operation by means of a sensor (not shown). Another method using a potentiometer, a magnetic system or the like is also known.
An iris encoder 226 is, for example, of a known type which detects the relation in rotational position between a rotor and a stator by means of a Hall element disposed in a meter which constitutes the iris driving source 224.
A camera signal processing circuit 228 applies predetermined processing such as amplification and gamma correction to the output of the CCD 221. The contrast signal of a video signal subjected to such predetermined processing passes through an AE gate 229 and an AF gate 230. Specifically, areas from which to extract signals best suited to exposure decision and distance measurement are set within the entire picture by the respective gates 229 and 230. Each of the gates 229 and 230 may be variable in the size of the area from which to extract the corresponding signal, or a plurality of gates may be disposed to constitute either of the gates 229 and 230, but the detailed description of such a gate is herein omitted for the sake of simplicity.
An AF (autofocus) signal processing circuit 231 generates one or a plurality of outputs relative to a high-frequency component of the video signal. Reference numeral 233 denotes a zoom switch, and reference numeral 234 denotes a zoom tracking memory. The zoom tracking memory 234 stores position information indicative of positions to be taken by the focusing lens group 201d according to different subject distances during a magnification varying operation. Incidentally, a memory incorporated in a CPU 232 may be used as the zoom tracking memory 234.
For example, if a photographer manipulates the zoom switch 233, the CPU 232 calculates a predetermined positional relation on the basis of the information stored in the zoom tracking memory 234, and drives and controls the zoom driving source 222 and the focusing driving source 223 so that the deviation of the output of the zoom encoder 225 from a target position to be taken by the variator lens group 201b and the deviation of the output of the focus encoder 227 from a target position to be taken by the focusing lens group 201d are made values of 0, respectively, so as to retain the predetermined positional relation.
In an autofocus operation, the CPU 232 drives and controls the focusing driving source 223 so that the output of the AF signal processing circuit 231 shows a peak.
Furthermore, to obtain a correct exposure, the CPU 232 drives and controls the iris driving source 224 so that the deviation of an average value of Y-signal outputs passing through the AE gate 229 from a predetermined value is made a value of 0 so that the average value of Y-signal outputs passing through the AE gate 229 becomes equal to the predetermined value.
FIG. 16 is a graph showing the field illumination ratio of the zoom lens. In FIG. 16, the horizontal axis represents image height, and the position of a value of 0 indicates the optical axis of the zoom lens (a position corresponding to an image height of 0). The vertical axis represents field illumination ratio, and the field illumination ratio on the optical axis is 100.
Curves 238, 239 and 240 represent specific characteristic examples, and any of the specific characteristic examples shows that the illumination in general becomes lower toward the margin of the picture (as the image height becomes higher). For zoom lenses, it is desirable that such illumination characteristic be approximately constant for any focal length.
However, if the iris of a zoom lens having a large zoom ratio is fully open, the illumination characteristic is difficult to keep constant and may vary according to the focal length of the zoom lens, for example, as shown by the characteristic 238 relative to a shortest focal length end, the characteristic 239 relative to a middle focal length and the characteristic 240 relative to a longest focal length end. Furthermore, the illumination characteristic varies according to the aperture value of the iris (the amount of variation is largest when the iris is fully open).
A zooming operation is performed in accordance with the manipulation of the zoom switch 233, and many image pickup apparatuses are arranged to perform zooming operation at variable speeds. It is well known that an image pickup apparatus using, for example, a seesaw switch as a zoom switch is arranged to detect the amount of depression of the seesaw switch by means of a sensor and execute zooming at a lower speed in response to a smaller amount of depression or at a higher speed in response to a larger amount of depression.
Some of the cameras of the above-described type as well as known still cameras have a so-called "mode select" function capable of variably combining shutter speeds and aperture values during photography. A photographer using such a camera may specify the desired photographic intention in accordance with the instruction of a pictorial icon so that, for example, an image shake can be prevented as completely as possible by using a high shutter speed during sports photography or a depth of field can be made as shallow as possible to defocus a background during portrait photography.
Another method called "aperture-priority AE" is known. In this method, a photographer manually sets an aperture value, and a shutter speed which can realize a correct exposure with the aperture value is automatically selected. If the aperture of an iris is generally reduced, a background or a foreground, particularly, bright spots are each defocused in a shape which reflects a polygon defined by the number of blades, such as a square, a hexagon or an octagon, whereas if the aperture of the iris is fully open, the bright spots are each defocused in a circular shape. Accordingly, if the photographer has a photographic intention to create such circular defocused spot images, the photographer needs only to set the aperture to a fully open aperture value in an aperture-priority AE mode.
However, the above-described type of image pickup apparatus has the following problems which have been desired to be improved. Specifically, with the recent improvement in the operating speeds of stepping motors or owing to the adoption of a lens group driving source which uses an electromagnetic type of actuator, called a linear motor, which can effect high-speed driving of a so-called voice coil type, the maximum zoom speed of the image pickup apparatus has been enhanced, and the time required to drive a variator lens group from a shortest (longest) focal length end to a longest (shortest) focal length end has been greatly reduced. As a result, if the field illumination ratio varies as shown in FIG. 16, particularly when high-speed zooming is executed with an aperture value which causes a large variation in the field illumination ratio, a variation in the brightness of a marginal portion of a picture becomes conspicuous, so that visual unnaturalness occurs.